The present invention relates in general to detecting the character of a medium in a sample chamber using acoustic signals produced by absorption of radiation; and more particularly concerns novel apparatus and techniques for detecting atmospheric pollutants using acoustic signals produced by absorption of wavelength-modulated radiation to achieve exceptionally high isolation of system error and noise, thereby facilitating detection.
It is known to measure trace gaseous constituents using acoustic signals produced by absorption of infrared radiation. In this technique the absorbed radiation heats the gas, inducing an acoustic wave which is detected by a suitable detector. The strength of the acoustic wave is directly related to the amount of radiation absorbed by the sample being analyzed. A method originally described by Bell, Tyndall, and others in the 19th Century was improved by L. B. Kreuzer, U.S. Pat. No. 3,700,890. A different method, where the incident radiation source is modulated at an acoustic resonant frequency of the chamber containing the sample to be analyzed, is described in Appl. Phys. Letters, 23, 633 (1973). Further elaborations of the art have used wavelength-modulated lasers to perform derivative spectroscopy, as exemplified by the paper of Hinkley and Sample, Applied Optics, 14, 859 (1975). A theoretical description is given by Sulzmann et al., Combustion and Flame, 20, 177 (1973). Still other art has used wavelength modulation of the optical source in a nonresonant spectrophone, e.g. C.K.N. Patel writing in Coherence and Quantum Optics, Plenum Press, N.Y., 1973, pp. 567-593.
To varying degrees, prior art opto-acoustic spectroscopy systems discussed above are susceptible to certain inaccuracies. For example, they are not self-calibrating; that is, there is no intrinsic method of insuring the relation between absorbed power and acoustic signal. If the radiation source wavelength is not fixed, no method is available to insure that the radiation wavelength is tuned to the correct value for the desired measurement. No method is available for detecting the fraction of the acoustic signal which arises from unwanted sources, such as the absorption of the sample chamber windows and absorption and spurious scattering from aerosols and the like.
Accordingly, it is an important object of the invention to provide an opto-acoustic spectroscopy system that overcomes one or more of the disadvantages set forth above.
It is another object of the invention to achieve the preceding object with an opto-acoustic spectroscopy system that discriminates against signals arising from unwanted sources.
It is a further object of the invention to achieve one or more of the preceding objects with an opto-acoustic spectroscopy system that is self-calibrating.
It is another object of the invention to achieve one or more of the preceding objects with an opto-acoustic spectroscopy system having the radiation wavelength tuned to the correct value for the desired measurement.
It is a further object of the invention to achieve one or more of the preceeding objects by utilizing a wavelength modulation frequency which is a multiple or sub-multiple of a resonant acoustic mode of the chamber containing the gas sample being measured.